In a conventional, fixed grid Dense Wavelength Division Multiplex (DWDM) optical network, optical signals are typically controlled by setting up a target for signal power, and then by measuring the total power of the optical signal, deriving an error between the target and the measured parameter, and then making adjustments to an actuator in single or multiple steps to achieve the target. In the fixed grid spectrum, the optical signals are spaced far apart from each other, or in other words, the grid defined by International Telecommunication Union (ITU) in ITU-T G.694.1 (February 2012), “Spectral grids for WDM applications: DWDM frequency grid,” the contents of which are incorporated by reference, is larger than the conventional signal bandwidth (BW) so that the optical signals do not overlap with each other in the spectrum. Hence, the Optical Channel Monitors (OCMs) that are typically used in the Optical Add/Drop Multiplex (OADM) nodes face no problem in measuring and reporting per signal power to the controller to adjust the actuator.
To address the continuous growth and the like, optical networks are evolving to flexible grid DWDM deployments. Note, flexible grid DWDM is now defined and described in ITU Recommendation G.694.1 “Spectral grids for WDM applications: DWDM frequency grid” (February 2012). Specifically, flexible grid DWDM allows a mixed bit rate or mixed modulation format transmission system to allocate frequency slots with different widths so that they can be optimized for the bandwidth requirements of the particular bit rate and modulation scheme of the individual channels. Also, in flexible grid DWDM, optical signals can be spectrally placed close to each other within a media-channel (that is also known as super-channel) in an OADM node to improve spectral efficiency. Disadvantageously, with flexible grid DWDM, the measurement of per signal power becomes difficult due to power contributions from neighboring signals. Specifically, when the optical signals are spectrally squeezed in Nyquist-spacing (center-frequency to center-frequency spacing between signals is equal to or less than each signal's baud rate), even with very high-resolution Optical Spectrum Analyzers (OSAs), it becomes difficult to measure signal powers without knowing each signal's spectral shapes, their overlapping conditions, and without ensuring equal power contributions from all signals within the media-channel.
With the optical channel monitors (OCMs) that are typically used in OADM nodes, which are basically limited resolution OSAs with resolution bandwidths (RBW) of 0.1 nm or more or less, for example, and suffer from aging that translates into spectral drift and inaccuracy, the task of measuring signal power becomes even more difficult. Hence, the conventional way of using a fixed and gridded BW to measure the total signal power, where the signals are guaranteed to be confined within that fixed BW, and not being spectrally overlapped with others, and using that signal power for per signal control does not work and ends up with erroneous results in a spectrally efficient flexible grid super-channel.
In evolving, dynamic optical network, where signal powers need to be adjusted dynamically to compensate for spectral ripple, gain tilt, Wavelength Dependent Losses (WDL), Stimulated Raman Scattering (SRS) or for fiber pinches, a signal controller has to react dynamically as well, and it is not obvious from the conventional controller techniques how that objective can be achieved to control individual signal powers that can equally be applied to signals, when they are closely spaced in the spectrum such as in Nyquist-spacing with different power targets and requirements, as well as, when they are spaced far apart from each other.